Accurate measurement of the BTU content of various hydrocarbon gases is extremely important in a wide variety of applications. The BTU content, or heating value of a gas, also known as its enthalpy, is the heat energy stored by the gas in the chemical bonds that are broken during combustion and the energy can be released as sensible heat associated with its temperature, and prior to combustion as latent heat associated with its state ("BTU" is an abbreviation for British thermal units). Measurement of the BTU content of natural gas is of particular importance in industrial processes, such as glass manufacturing and heat treating. Instruments for measuring BTU content can typically utilize a calorimeter or gas chromatograph or a gas sensor comprising a coil of a fine platinum wire coated with a catalyst, such as alumina, to form a bead. In U.S. Pat. No. 5,012,432 to Stetter, et al., which is herein incorporated by reference, the present inventors demonstrated that the overall dimensions of the sensor's bead are often less than a cubic millimeter.
During operation of the instrument, the sensor is heated by passing current through the platinum wire. When a combustible gas is contacted with the hot catalyst on the bead surface, the typical hydrocarbon gas (HC) reacts to produce heat (.DELTA.H) according following general equation: EQU a[HC]+bO.sub.2 .fwdarw.cCO.sub.2 +dH.sub.2 O+.DELTA.H (1)
where a,b,c,d are constants. The amount of heat produced, .DELTA.H, is determined by the enthalpy for the particular hydrocarbon gas, the degree of combustion, and number of molecules reacting or concentration ([HC]).
The platinum wire in the sensor can also function as a resistance thermometer, that is, its resistance changes with temperature. When the combustible gas reacts at the catalyst surface, the heat produced (.DELTA.H) causes an increase in sensor temperature (.DELTA.T) that can be related to the sensor's heat capacity (C.sub.p) and the heat released according to the equation: EQU .DELTA.T=.varies..DELTA.H/C.sub.p ( 2)
Where ".varies." is the fraction of the released heat, .DELTA.H, that goes into heating the sensor which has heat capacity, C.sub.p. The change in sensor temperature, in turn, causes a change in resistance of the sensor's platinum wire. The change in resistance of the wire is monitored by placing the sensor in a Wheatstone bridge circuit with a compensating element (passivated and matched) and two known resistors. Thus, small changes in temperature are detected as an imbalance in the resistance bridge circuit. This relationship is usually expressed according to the following equation: EQU V=K.times.[HC] (3)
where V is the signal from the sensor corresponding to the imbalance in the bridge in volts. When the sensor is operated under constant conditions, V is dependent only upon the relative amount, or concentration of the hydrocarbon gas in the sample as illustrated in equation (3). K is an instrument constant obtained through calibration of the sensor with a known concentration of a gas in air and varies with the type of gas being detected and the combustion conditions.
A microcalorimeter utilizing the above-described technology to measure the heat content of natural gas is disclosed in U.S. Pat. No. 5,012,432. A microcalorimeter is a computer-controlled instrument which samples a fixed number of moles of sample gas. The Stetter et al. microcalorimeter offers convenient portability and a low production cost while retaining high performance, which are features that are not available in many other prior art calorimeters or equivalent BTU measurement devices. However, these positive features of the device are often outweighed by the device's susceptibility to numerous sources of error. In particular, inaccurate output signals used for calculating BTU content may result due to only slight changes in conditions within the sensor chamber. Further, accuracy of the device varies with the composition of the sample gas, and age and composition of the sensor being utilized.